The proposed work is designed to characterize in more detail the rate limiting step in bile acid synthesis. The enzyme involved is 7 alpha-hydroxylase and is under the control of bile acids returning to the liver in the enterohepatic circulation. Studies are designed to determine the mechanism of control of bile acids on 7 alpha-hydroxylase. Additional studies are listed to investigate the uptake mechanism of bile acids from portal venous blood into the liver in normal and cholestatic conditions. Other studies are described to better understand the mechanism of hepatic oxidation of bile acid precursors into the bile acids, cholic acid and chenodeoxycholic acid.